Customers regularly joked that it was right twice a day
. Employees walked past it, mostly ignoring it — except to make sure they didn’t brush against it and smudge a sleeve.

These days customers and workers pause to admire the restored timepiece, and visitors often ask about its history.

Six months ago, no one at E.B. Horn knew the full story. Then, owners of the venerated Boston jeweler, which traces its roots to the 19th century, decided to restore the long-ignored antique and research its provenance to mark the store’s 175th anniversary this year.

Turns out the clock is a rare 19th-century astronomical regulator and was created by the store’s founder and namesake in 1839, the year he opened the store.

While officials at Massachusetts-based Skinner auctioneers say putting a price on the clock would be complicated, a locally made regulator of similar vintage drew $535,000 at a 2010 Skinner auction in Boston.

Experts say that these astronomical regulators were an upgraded version of standard pendulum timepieces, and New England clockmakers were at the forefront of the innovations.

The clocks were valued at the time because of their accuracy and were relatively expensive so not plentiful, with perhaps as many as a couple thousand in the United States at the height of their popularity — which faded after the rise of the more accurate quartz technology in the late 1920s.

Besides being relatively uncommon, astronomical regulators played a major role in the beginning of the standardization of time in the United States in the late 1800s, a development that would bring improvements to the efficiency and safety of train travel and boost government, business, and scientific exploration, all of which benefited from the ability to precisely measure time in smaller and smaller increments.

“For quite a long time, especially in the 19th century, these clocks set the standard for time,” said Robert Cheney, director of clocks, watches, and scientific instruments at Skinner. “Largely, they were used to help government and big business keep accurate time. But this clock at E.B. Horn was made available to the average person from the neighborhood to set their watches by.”

While the clock’s background surprised staffers at E.B. Horn, one customer and friendly rival was not surprised. Paul Calantropo had been telling store employees for nearly 40 years that they should take a closer look at the clock or let him do so. Last fall, Calantropo got his way.

“You can’t blame them for not being fully aware of what they had there,” said Calantropo, a clock maker and restorer who has been the keeper of Greater Boston’s most famous tower clocks, including those at South Station and the Custom House. “It hadn’t worked for so long. And you would likely have to specialize in clocks to understand this one’s history.”

Calantropo, who owns Paul Calantropo Jewelers, a couple blocks away from Horn, maintains a small clock museum in his shop.

“I recognized it from the very beginning as astronomical and rare,’’ he said. “So every couple of years I’d go in the shop and tell them, ‘You know, I’d love to take a look inside,’ and ‘I can fix it.’ ”

Horn’s owner admits that Calantropo’s reminders of the clock’s value served as a prod, but the final decision to do something with the clock was triggered by the store’s approaching anniversary.

“I started working here in 1970, and I knew that [the clock] had some history with the shop, but not much more,” said Michael Finn, whose parents purchased E.B. Horn in 1947.

Finn says Calantropo’s words went unheeded for so long because no one believed the clock could be fixed. “Paul was persistent, though, and with the conversation being about the history of the clock and the genre, I thought with a major anniversary coming up in the history of the store it was worth giving him a crack at it,” he said.

Last September, Calantropo
began his repair work on the clock itself while a cabinet-making team from the North Bennet Street School restored the case. Finn also enlisted the help of Cheney, the Skinner clock expert, to uncover the timepiece’s history.

What Finn learned over subsequent months was that E.B. Horn’s was an astronomical regulator, so named because they were considered as much scientific instruments as clocks and were initially used primarily by observatories to precisely track the movement of celestial bodies.

Two things set these clocks apart from their pendulum peers: They were typically constructed of materials less sensitive to changes in temperature and were engineered to run continulously while being wound, unlike other clocks, which stopped during winding.

Regulators were able to keep accurate time to within a few seconds a month, compared to within about a minute for traditional clocks.

Finn also learned that the piece had been built by store founder Edwin Booth Horn in 1839, according to records and pictures found by Cheney. Horn constructed the clock while working as a machinist for Daniel Davis Jr., who ran a Boston invention mill, the 19th-century equivalent of a technology think tank. Davis received one of the first patents issued in the United States, for a process to apply color to daguerreotypes. He also employed Elias Howe Jr., who invented the first lock-stitch sewing machine.

Before the year was out, Horn would launch E.B. Horn jewelers, and when he left Davis’s employ he took his clock with him.

“When you get into the history of [the clock] and of the era when it was most popular you learn so much about how people lived and what they valued,” Cheney said.

One thing they valued was accurate time, he said.

When Horn built his clock, time had not yet been standardized in the United States. Average Americans set their clocks either by the movement of the sun and stars or by more accurate pendulum clocks, often found in public settings or businesses.

Access to astronomical regulators typically fell to the well-heeled, who could afford their own, or to those lucky enough to have one at their places of business. A few jewelers, like Horn, had the clocks hoping to lure customers. But again, they were rare enough that at the time Horn built his clock there were probably only about a dozen similar in all of New England.

Standardization of time tended to be a local affair, and there were in fact dozens of time zones across the nation. But by the mid-19th century, railroads had noted the importance of standardized time zones and accurate, synchronized time for the safety of trains on shared tracks.

The ability to keep precise, incremental time was also becoming increasingly important to scientists other than astronomers.

Decades before Horn built his clock, Harvard University had already commissioned several astronomical regulators for use in its observatory, said Sara Schechner, David P. Wheatland curator of the collection of historical scientific instruments at Harvard.

The school acquired its first astronomical regulator in 1675, Schechner said, and later purchased several more, each a little more advanced and accurate than the last.

By the early 1850s. following a terrible train crash on a rail line shared by Boston and Worcester, Harvard, in cooperation with Boston clock maker William Bond Sr., who was also director of Harvard’s observatory, began using astronomical regulators to distribute the time at set internals via telegraph to regional government buildings and railroads so they could synchronize their clocks.

At first Harvard’s service was free. But from the early 1870s to mid-1890s the school charged a nominal fee, raising more than $2,500 in sales.

The practice of synchronizing time spread around the nation. By the end of the 19th century, American and Canadian railroads had created a standard time-zone system, which Congress adopted in 1918.

The drive for ever greater precision in the measurement of time is evident in Horn’s clock.

Calantropo, who took three months to restore the regulator, starting last fall and finishing in late December, said he was moved by its craftsmanship and by Horn’s innovations.

“Weights were common balances for clocks back then. That this fellow Horn used a spring mechanism speaks to a certain care and that he wanted it to be just a little better than the best, more accurate to the second,” Calantropo said.

Where Harvard and other observatories in the nation sold accurate time, Horn apparently “gave time away by letting regular people come in and set their watches by it,” Calantropo said. It wasn’t all magnanimous. Horn hoped to gain repeat customers, but it was still a public service with the era’s best equipment, which most jewelers couldn’t provide.

So it seems fitting to Calantropo that he got Horn’s clock restarted for the store’s anniversary.

“I like to hope Mr. Horn would appreciate that his clock is working again to mark the date” for all, said Calantropo.